ARCHIVED: In Mac OS 9 and earlier, what is Disk Caching, and how do I use it?

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The time it takes a hard disk to fetch and retrieve requested data can
be described by a variety of measures, such as average seek time and
data interleave. However, speed is always limited by the physical
facts. A disk drive is a fast-spinning platter with a flying
recording head that looks for specific bytes among millions
of bytes. RAM, on the other hand, has no moving parts, and
fetch and retrieval functions are performed so much faster that access
speeds are measured in nanoseconds instead of milliseconds.

Mac OS uses the speed of solid state RAM to alleviate
common data bottlenecks. The Memory control panel can
allocate an area in RAM for storing the contents and results of a
previous hard disk search. The cache provides an alternate area for
the system to check before going to the disk. During subsequent
searches, if the system finds requested information in the cache, a
disk search is not required, saving potential hard disk seek time and
speeding up the retrieval process.

Operations that require repetitive pieces of information to be loaded
and unloaded are ideal for cache storage. Because Mac OS builds and
runs its applications by loading and unloading resources in small,
discrete chunks of code, it is an ideal system for caching. However,
operations that require accessing large chunks of information spread
out unevenly over the hard disk defeat the purpose of a cache. Large
word processing or database systems may actually slow down
performance because the system must check both the user-definable Disk
Cache and the disk buffer.

As a rule of thumb, you want to set the Disk Cache at the ratio of
32KB for every megabyte of physical memory in
your Mac OS computer. Thus, if you have a 4MB system, a cache setting
of 128KB would be normal. For an 8MB system, a setting of 256KB will
be normal.

Note: Settings above 512KB may degrade performance on
many older PPC computers and all 68k Macs. For example, if you have
32MB of physical memory in a Quadra 660AV, the highest setting you
would want to use is 512KB, even though with the 32KB per megabyte
rule you would think it should be 1024KB. On newer PPC computers,
especially any running Mac OS 8 or higher, you can get better
performance out of higher cache settings. The default setting is
usually best. To set the default setting, from the Memory
control panel, click Default.

This is documentaapzin the Knowledge Base.Last modified on2018-01-18 08:56:42.